There is a program called "IRFANVIEW". Entering that into Google will yield the necessary results. the top result will have a downloads link on it. the program is free ware and has no malicious software associated with it. The program and associated drivers only consume 1.38MB of space, so it's a fairly small program by todays standards, and should run flawlessly on 99.9% of computers.
Here is their homepage:
http://www.irfanview.com/
Once you have the program downloaded and installed (takes only a few minutes). run the program.
-Once in the program along the tool bar on the top select: file>Open
-Choose the file which you want to modify.
-From the tool bar along the top select: Image>Resize/Resample...
-At this point, a window pops up and you can choose exactly how you want to change the image. You typically want to keep "preserve aspect ratio" checked (lower left of the window). This keeps the proportions of the picture so it doesn't get stretched taller or wider.
-I typically use the "some standard dimensions:" sub-window and just click on "half", but there are also standard dimensions by pixels. (ie. 640X480 ETC.)
-There are two methods to choose from in the lower right (resize or resample). not sure of the difference really, but both will result in a smaller file size.
-I think maximum file size for JPG and BMP images on SCCOA is 200kb. Typically using the "half" button will result in a significant file size decrease.
-There are plenty of features (rotate, flip, color depth ETC.) Play with them and figure them out if you want to.
-Once you are done modifying your image, Select from the tool bar across: File>Save As
-In here you can choose where and how you want to save the image. On the bottom there is a pull down menu called "save as type". I believe the default for this is a BMP file format. If you're looking to post pics on SCCOA, or trying to conserve disk space, don't use this!!!
-Select JPG from this pull down list. When you do this, another window will open up for "save quality". I typically just choose 100%. If you select "save quality" as 100% for a JPG, the file size will typically be around 10% of the size it would be if you saved it as a BMP (2.95MB* Vs. 22.7MB*). Saving it at 80% quality will shrink it even more (748KB*).
*Sizes for the particular picture I am working with for this example.