A Freeware Program called IRFANVIEW has been available for many years and the creator keeps improving it. It's relatively small and easy to use. For posting, I usually resize to 4" X 6" at 72 DPI, but many forums will now take larger. 72 DPI is about the smallest that displays good on TV or Computer Displays. It is my "GOTO" for sizing, but it also works well to create slide shows of every photo in a directory, then repeats until stopped. I've been using this program for the past 25 years for basic editing, sizing, and slide shows. Irfan is the first name of the developer. For safety, I'll only download Freeware from CNET. Be certain to download both the program and the Divix/Codex file.
Charley