If you have a Windows photo program or other brand of photo processing software, or even one supplied with your digital camera, you may be able to use it. Different providers use different terminology, "resizing" being one, but most work similarly.
Get your photo into such software, then click on the File menu button far upper left corner. Choose "Save as". It is likely that somewhere in the "Save as" window that opens is a button, or a link to one via an "Options" button, that allows you to reduce the size or resolution quality of the image, although some simple image editing applications ("Paint" for example) don't include resizing capability. Some let you pick a number, but many use a slider bar where you move to a percentage of size or quality less than 100%. After adjusting that setting, back at the "Save as" window you make sure that the "Save as Type" field has "jpeg" or "JPG" selected, since jpeg format is commonly acceptable for uploading to sites like the Forum. Then click on "Save" or OK to save your changes. The resulting image copy should be fine for uploading to the Forum as long as it is less than 1MB.
Usually, the first thing I do before all that is make a copy of the photo in question in the computer's photo album (My Pictures or whatever). The copy ends up in the album right next to the original. I then only edit the copy, so when it is "Saved as", I have both the original at full resolution and a a copy at a lower resolution for future use. In theory, when you pick "Save as" from the File menu rather than "Save" after editing an image, the original remains automatically untouched in the album alongside the new copy, but I've never trusted that theory with all my programs.
Joel