It's easy enough to add a caption just using Microsoft paint. Just open the image for editing (CTRL+E) then select the text box tool from the side bar (the A icon) drag your text box where you want your caption then type it in and hit save. Note: you will not be able to edit the text box after you if you come back later so keep a uncaptioned copy of the picture in case you wish to redo it. Also Microsoft paint has limited font options so if you want to play with different fonts and styles of text then create the text box in Microsoft word then copy and paste it into paint after you've finished. Before pasting a text box into paint go to the image tool bar and untick draw opaque if you want to be able to see the picture behind your text.