And remember that your pics need to be in 2's (32x32,64x64,32x64,128x128,128x256 etc...)
This is how I do it:
Lets say you want a pic around 256 x 256
Find the pic you want ....cut and paste it as a "new image"
Look at its properties and see its size
Doesnt matter what size it is really but lets say its 244x177
If its under 256x256 follow this:
Make a new image sized 256x256 and paste the pic in the middle and save
If the paste was larger than 256 (lets say its 288x270)
The "new image" that you have can be resized but it wont be exactly 256x256
So do what others stated above.....
Resize the pic making sure the maintain aspect ratio of 1:2 is selected and resize the highest number to 256 and the other number will change automatically and the image will KEEP the proper aspect
After the image is resized to within/under(less than) the size you want (usually 64x64,128x128,256x256 ,512x512 or even 1024x 1024)
Make a new blank page 256x256
copy to clipboard the image you resized and paste it on the blank 256x256 sheet and it will fit