Try this

https://noscrubs.net/discussions/how-to-make-a-guild-mark-transparent-with-photoshop

It's really much simpler if you can use GIMP.

1. Download old version 2.6.11 of GIMP at https://download.gimp.org/mirror/pub/gimp/v2.6/windows/gimp-2.6.11-i686-setup-1.exe

2. Open your (preferably already transparent) image in GIMP.

3. If it's not already a transparent PNG or some other format with transparency then you will need to edit the image to remove the background. I can't teach you how to do that so check google for some guides - but it's usually enough to use the Fuzzy Select Tool to remove white backgrounds.

4. Save the file as mark.bmp

5.When the Save as BMP window pops up, click the + sign for Advanced Options and make sure 32 bits is selected and click Save button.