change default file format when saving pictures by MS Paint

Take a look at this Windows Forum Post. What it does is create a quick access link that will default the Save File dialog to jpeg, you would then click this icon to save your file as jpeg.

From Link:

Open PAINT, then click the BLUE drop down box in the top left hand corner of the screen (this is the box that gives the drop down options for saving a file), now click SAVE AS and a selection of save as formats including JPEG will appear to the right, move your cursor over the JPEG option and RIGHT CLICK (not left click), three options will be presented, click (select) the top option which says ADD TO QUICK ACCESS FOLDER. This creates a new ICON at the top of the paint screen next to the DO and UNDO arrows (icons).

In addition to the answer by mark-hall:

As the author zdd asked for a keyboard shortcut to save the file as jpeg, you can now use keyboard shortcut ALT + 4 (if the icon is at the 4th place).