Arthur - when resizing pics, try to keep the aspect ratio, this way they won't become stretched or look weird.
for info on keeping images sharp, and correct contrast, try using good-tutorials.com and search for techniques relating to this.
k750 theming is confusing enough to start on, k700 maybe easier as it has less screens to work with. good attempt nonetheless.
when making the theme, keep on previewing it and see how functional it is. also check out other's themes to get ideas and info on hot to make the best out of the space your working with.
a few refferals for your info:
clean functional themes: spawn, simon, gseth
technical themes: patrick, BBT, TT
good balance: augustborn99, zleepy, jasnasto
take a look at how their themes contrast and different approaches to them. and yes joining the TMC will help a LOT.
finally, i think i've said too much