GIMP is a free image-editing software that has many of the features of Photoshop. Many people use GIMP to do digital scrapbooking and art journaling as well as other kinds of artwork. Adding a paint splatter to these types of images is a popular way to add a grungy look to the artwork. It's easy to do that in GIMP.
Visit a website that has free GIMP brushes available for download and download one or more (or a set of) brushes in the shape of paint splatters (see Resources).
Unzip the brush files and save them to the "Brushes" folder inside the "GIMP" folder on your hard drive. The GIMP folder will have a number after the name that matches the software version number. Make sure to do this when GIMP is not running, or restart it after copying the files so that the program will have access to the new brushes.
Run GIMP and open the image that you want to decorate with the paint splatters. Make a new layer using the plus sign icon on the Layer Dock.
Select the Paint Brush tool and then select the paint splatter brush from the brush set below the toolbar. You may have to scroll down to see the new brushes you installed. Choose a colour using the colour chooser.
Hover the brush over the image window. You will see an outline of the brush in the size that is currently selected. If this is too big or too small, adjust the size using the slider in the brush options section, between the toolbar and the area that shows all of the specific brush shapes that are available. Check the size by hovering the cursor over the image again, and click on the image when you have the right size and location. You have now added a paint splatter to your image.
If you add each paint splatter to its own layer, you can rotate the layer if desired to give the paint splatter a different look. You can also give it a more washed-in aspect by adjusting the opacity of the layer in the Layer Dock.