Try
http://imageshack.us/ It has a photo size option.
Like this--
Once you go to imageshack you will see a browse button to browse to the file you want.Then below the browse button check the image size box then select the size of the photo you want to display.
(The photo above is 800x600)
Then hit the "Host it" button. Then you will be taken to another page. If you want it to be displayed directly on your forum post,highlight and copy(control+c key)the link for- "Hotlink for forums" -and paste it to your forum post(control+v key). Whamo your photo magically appears.