Opted to call my sprite editor "SpriTed", to go along with the Blitz editor which is called "Ted".. I'm imaginative, like that..

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The Amiga's Workbench (or at least, the v2.04 that I'm running) doesn't include a right-click menu.
Instead, you have to install a separate tool ToolsDaemon which lets you assign programs to the top-bar menu.

Open the ToolsDaemon preferences, drag your wanted tool into the window, give it a name, save, sorted.

Once done, you can click an icon in a window, (eg, your sprites.iff file) then select your tool from the menu at the top.
The selected icon gets passed as a parameter when the program is executed, and all is golden.
With this functionality, I spent the next hour or so trying to work out exactly how a Blitz app would receive the parameter.

If NumPars>0 then RequestedFile$=ParPath$(1)+"/"+Par$(1)
(Though you should beware incase ParPath$(1) is a drive, in which case it'll end with a Colon.)

It seems to do the trick, and now means I can create a template with a sprites.iff file, copy the template folder to create the next game, right click the sprites.iff, edit the sprites, and then spit it back out again, all from the Workbench interface.
Gud gud!

There are still extra things I need to add to the editor. Scrolling images, copy+paste, Fill, that sort of thing, but for the meantime it's at a point where I'm just happy that it works!

Note : When I say Sprites, I'm referring to small 16x16 images, very much like JSE's symbol editor.
The images are 32 colours, but when used as actual hardware sprites only the first 3 colours count, so there'll be a LOT of planning ahead necessary!