A quick and dirty solution would be adding rewrite rules for those existing icons, sending the request to your own icons instead.
Just add the rewrite rules to your web.config.
I'm looking to try to customise the episerver form element icons! (see below)
As you might notice, I've already replaced three of them. This is because they have their own custom Model.cs file (so it was no diffrent from any other non forms block - update SiteImageUrl in the model)
However, the rest (in blue) do not have their own custom model file!
I'm thinknig that this is achieveable in the same way as customising the form element views (by putting them in a particular file structure in the solution). EG (for views) Views / Shared / ElementBlocks / custom.ascx files
I cannot find any documentation for this though and so, I'm having real trouble with it!
As an exmaple the Text elements icon src default is /EPiServer/EPiServer.Forms.UI/4.24.2//ClientResources/epi-forms/themes/sleek/images/contenttypes/textboxelementblock.png
Has anyone done similar in the past or have some steer for me?