While not as robust as the old Microsoft Photo Editor (see below), Picture Manager is a great free program that is usually pre-installed on Windows computers. If you are unaware of Microsoft Office Picture Manager, your images are likely opening automatically in Windows Picture and Fax Viewer. Right-click on any jpg or gif image you want to open and choose "open with - Microsoft Office Picture Manager" and you will be able to perform quite a few editing options like cropping, adjusting color qualities, removing the evil red eye, and resizing. However, as soon as you close the program, pictures will continue to open in Picture and Fax Viewer. To make the change permanent, when you right-click on an image and choose 'open with', click on the 'choose program' option, then choose Microsoft Picture Manager from the list. Be sure to check the box next to "always use the selected program to open this type of file". You will need to do this for both both jpg and gif images. For bitmap images (bmp), choose to open them in Paint. I will be uploading a video tutorial on this topic soon, but you can contact me if you have any questions in the mean time.

Your students probably know how to use Paint, but, since you might not know all the ins and outs, here's a tutorial .
Paint is a great throw down activity for when the internet or server goes down. You can even justify it with a few TA TEKS. Have the students paint a picture using however much time you need to give them. Create one of your own, too. Demonstrate for the students how to save a picture you have created. The default extension for Paint images is *.bmp (bitmap) which are very large files. Right-click on the image you saved and show the students how to determine its size. Now, reopen the bitmap image and show the students how to save as a gif and jpg by changing the "save as type". Compare the file sizes of the 3 images. The students will be amazed at the difference. Paint images should be saved as bitmaps only if further editing will be done.

In the 'old' days, bmp images always opened in Paint. Now that is not necessarily true.
If you have coloring sheets, diagrams, connect-the-dots, etc. that you want students to open in Paint and they don't, you will have to associate the .bmp extension with Paint. To do this, right-click on any file with the .bmp extension. Instead of choosing Paint from the list of programs, click on 'at Choose Program' at the bottom of the list. Now, click on Paint, but before you click OK, be sure to put a check mark in the box next to 'Always use the selected program to open files of this type.'