If you can look at the pictures on your computer, then you are opening them in SOME program, whether it came on your pc or was installed when you installed the drivers for your digital camera from the cd rom that came with it.
Open a picture and look across the top for something that says "image". I have used several programs to open pics and there has always been an "image" in the top taskbar. Under image should be the option to resize. If you see something that says "lock aspect ratio" put a checkmark in the box and the picture will reduce in size proportionately (width and height). It doesn't really make any difference if you reduce it in size by pixels or dimensions. After it is smaller right click on it and choose properties to see if it is small enough to post.
Reducing the picture in size may reduce it permanently. In other words, it may lose enough quality to make a poor printed version. Once you hit "save" it's too late. If you think you may want to save the original pic, then copy and paste it within the program and work on the copied one.
Hope this gives you something else to work with.