Dissolve

Dissolve is a small-sized extension specifically designed for Paint.net, which gives you the possibility to make a picture look like it's dissolving on top of another one.
It enables you to adjust the amount of grains. The tool can be handled with ease even by those getting to know this image editing application.

Simple integration with Paint.net

There is no actual installation pack involved. In order to get it up and running, it's only necessary to copy the downloaded DLL file to the "Effects" folder of Paint.net's installed location. It also works with a portable version.
If the program is currently running, you have to exit and relaunch it to complete the setup and gain access to Dissolve via the "Effects" menu and "Distort" submenu. It's marked by a tiny icon that resembles a puzzle piece, which indicates all Paint.net plugins from external sources.

Load a picture to dissolve

After loading the base photo, you can use the file browser to locate and select a new image to overlap it. Before committing modifications, you can tweak the level of intensity to make the dissolving effect more or less obvious by moving a slider between 0 and 100.
Alternatively, you can directly enter the numerical values or click on two arrow buttons to increase or decrease the amount of grains. All of this can be done while previewing the new photo, so the tweaks are made in real time.

Evaluation and conclusion

Although the utility hasn't been updated for a long time, it successfully integrated with the latest edition of Paint.net in our tests. It didn't cause it to hang, crash or display error messages.
On the other hand, Dissolve doesn't leave much room for customization, especially for experienced Paint.net users looking for versatility.