Declining cake beyond ones skill level?

Hi,
I am curious to find out how others handle a situation where someone requests a cake to be replicated from a picture that you clearly know is beyond your skill level. I am not referring to the etiquette that comes with putting your own twist on another's design in order to make it your own. I mean if you know that even if you change colors etc that your skill level will just not enable you to deliver a cake up to the standards of what the person is looking for in the picture they give you.
Do you decline completely? Tell them your skills arent there?

My situation is that I am fairly new to cake decorating (less than 25 cakes that I have done for family and friends). On my personal facebook page I have posted pictures of cakes I have done and have gotten great reviews (from friends and family of course, lol). My cousin posted a cake i did for her daughter on pinterest and it got almost 500 repins in less than 24 hours and while I thought that was so cool, I am extremely critical of my work and am never satisfied with my cakes. I know that I have a lot to learn and practice so I think being hyper-critical vs overconfident at this point can only help me. I would never want to present someone with a cake that looks like a super crappy version of what they had in mind. While do I think that for only having done the small number of cakes that I've done that i really have a knack for it I know that some of the seemingly simple yet hardest things are what I really need to work on (super crisp corners, perfect fondant work, piping, etc).

So with that said, I agreed to make a cake for an engagemnt party for friends of my husband. They have seen my work and know (or at least i thought they knew) my skill level, however when sending me pics of cakes they want for the party they are from world renowned cake artists. Cakes that probably seem simple to someone who might not make cakes but are quite difficult to achieve the high quality look. Those simply decorate yet perfectly executed cakes. Even though i'd be doing the cake as a favor and to get more practice i do not feel that my skills are up to that level. Can anyone help me with how I can go about saying this?

Thanks you all for your responses, I definitely appreciate all of the advice. Up until now all of the cakes I have done have been either birthday cakes or baby shower cakes. While I've followed the general theme of the parties I've pretty much had more creative control of the cakes thus being able to switch things up if a problem arose. While I've generally liked the cakes I've done I know that I need to work on not having rounded edges, cleaner fondant work, piping, etc. To my friends and family who don't decorate cakes they are thrilled but for a person looking for a high end cake for a fairly expensive event they are definitely no where near that level.

With the cake for this party they want a very specific design and as soon as I saw the picture I just felt I could not make a cake as nice as what they'd be expecting. The pictures of the cake they want had very crisp corners, super cleanly executed, professional level for sure. The top tier has a very detailed design with some paisley and scrollwork but i can't quite make out the technique used. It either looks like the design is hand painted on (but i dont think so) or a technique where it looks like one seamless piece of fondant vs the usual tier covered in fondant with the design elements then attached if that makes sense. Unlike for example, a zebra print where one can take the stripes, lay them atop a rolled out piece of fondant, roll them together and get the appearance a seamless piece of zebra print fondant this design was too detailed for that to have been the technique (I think?). I think what it all boils down to is that the cake they want is so well executed and obviously someone elses artistic vision and I don't think I could recreate that at this stage in my decorating.
My husband tells me I am just over thinking it and that if I looked at the cake step by step it wouldn't be that hard but I don't know.

Thank you all again for your advice, I actually did send an an email this morning similar to what cakesbycathy just posted. He wrote back and said that he is totally fine with my changing around whatever I wanted, that the pic was just for me to use as an inspiration. It is more the color scheme they would like and overall feel but that if the cake turns out same quality as my other cakes they are totally fine with that knowing that I'm not a pro by any means yet and they like what they have seen me do so far. I will take the advice offered here and will definitely practice to be sure whatever I end up putting on the cake can be well executed. Thanks again!