If you don't have a resizing program, or you can't seem to get the dimensions right.... I found an easy way around all of that.

Send any picture you have on your computer to your 8700 via email. Open attachemnt, click the wheel and zoom in as preffered (till it fills out the screen usually), then click "enhance". The picture will then adjust to be as clear as it was originally. Save the picture, then set as homescreen image.

PS. Pictures sent from you computer should be greater than 320x240 for "enhance" to work.