Photo Size
To clarify Gary's comments, the 800x600 resolution works for the gallery, but it needs to be much smaller (350x250 or thereabouts) to attach a photo to a message. And you MUST save it in a .jpeg/.jpg or .gif format in either case, NOT a .bmp format, which is many times bigger in size for a given photo.
As mentioned, all you need is an image viewer/editor program, such as MS Photo Editor or Irfanview, which is in the download section on this site. Most digital cameras come with an image viewer/editor program (my Pentax came with ACDSee) which should also work. Simply open the photo in the image viewer/editor, find a choice such as "Image/Resize" (in MS Photo Editor), "Edit/Resize" (in ACDSee), or "Image/Resize/Resample" in Irfanview, and set the number of pixels to the desired count. Save it (as a new name, otherwise you just changed your original photo; I just usually add the new pixel count to tell me it is a lower resolution photo) and then post the saved image. Easy!