what photo editing software are you using, if you are using photoshop, it is easy.
Go to image>image size>constrain proportions and then plug in the diminsions 800x600 and it should resize the image for you. do a "save as" and then upload that image to your image host and you should be good to go.
TTORA forum is the best Toyota off-road club around. We are nation wide with chapters in most states. Come in and discuss Tacoma, 4Runner, Highlander, & TRD models.