with making mod (size etc.)

i am wondering if size matters in mods, i am not going to go crazy and make is 1600*1200, but is there a size limit or something.
also question: can you change pictures by putting your own pics and call it as the same thing?
someone please help me...i am new to mods and i need to just understand anything that is importmant.

i have made some mods (took me 4 hours!)
and i just want to make sure everything about them is ok.

Yah there is a size Limit, It's 96 by 96..

Do u mean that if ur gonna make kill badges it has to have width-96 and height-96?