Do you have a good editor for resizing/cropping your images? If not, these are free online graphic editing tools you could use to standardize the size of all your images:
http://www.picresize.com/
http://www.resizr.com/
http://www.webresizer.com/resizer/
https://resizemyimg.com/
Each one is a little different, see which one you are more comfortable with.
For a more full featured photo editor that allows you to create banners and header graphics from images try:
http://pixlr.com/editor
These tools meet the needs of most beginners.
