For anyone interested in custom particles for the "Particles" effect, there is a workaround to achieve that until the capability is built into Video Editor. The workaround is admittedly a bit clumsy, but does work as the attached brief video will show.
The Particle images used by Video Editor are stored as 16x16 and 128x128 .png images files (named "16.png" and "128.png," respectively) in each of 100 subfolders (named "particle_001" thru' "particle_100"within the folder C:\Program Files (x86)\AVS4YOU\AVSVideoEditor\Data\Particles. Unfortunately Video Editor does not "see" additional similarly constructed and named particle files created and added to the set, but it will use your custom images substituted for the built-in images in one of the original set. The cumbersome bit is that, when editing a Particles effect, the custom image doesn't appear in the dropdown list; instead, you must remember that you substituted your custom image for, say, the first one in the list (a white snowflake), corresponding to the .png image files in the folder
C:\Program Files (x86)\AVS4YOU\AVSVideoEditor\Data\Particles\particle_001.
My demo can be duplicated as follows:
1) Use any photo/image editing software to create any image you want as a .png image file in sizes 16x16 pixels and 128x128 pixels. I use Photoshop as there does AVS4YOU's Photo Editor does not seem to support scaling down an entire image to arbitrary pixel dimensions.
2) Rename folder C:\Program Files (x86)\AVS4YOU\AVSVideoEditor\Data\Particles\particle_001 to anything convenient, just to preserve the original particle
3) Create a new replacement folder named "particle_001"
4) Into this new folder, place the 16x16 and 128x128 .png images, naming the files "16.png" and "128.png," repectively
5) Start up Video Editor with a new project
6) Place a particles effect in on the Video Effects line
7) Double-click on the effect to open the Video Effects Settings window
8) Click the down arrow beside "Particle:" to open the particle choice list, click the snowflake at upper right in the array of available particles. At this point, the shower of particles in the preview window should change to your custom image.
9) Click OK do dismiss the Video Effects Settings window.
10) That's it. Play your video to confirm that the effect works as you intend. It will work exactly as do the built-in effects.
For my demo, I made a "particle" from a photo of my dog and substituted it for the original "particle_001" images.
Since modified images actually present in the particle_### files don't appear in the list when the particle-chooser dropdown is accessed, it seems likely that a set of copies of the default images is "hardwired" into the program code itself. It would seem a fairly simple matter to include the capability for an arbitrary number of custom particles in Video Editor if instead the editor canvassed the available particle_### files to create on the fly the list of actual images when the dropdown particle list is accessed. I should think it fair play to leave it to the user to create the particle image files and properly store them for access, tho' a rudimentary capability to create and store custom particles from images loaded into a project would be a handy inclusion within Video Editor itself.