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[fevredsuk]

You could just create a rank with the required image and attach it to the subscription usergroup.

e.g. create a usergroup subscriber
when a member donates/subscribes the subscriber usergroup is added to that member. (their primary group is not changed)

also create a rank
Number of times to repeat rank: 1
Usergroup: subscriber
minimum posts: 0
stack rank: Yes
display type: always
user rank file path:
html text: <img src="url of the image">

update usertitles and ranks

Once the member subscribes the image should be added the next time your promotions scheduled task runs

Ranks show below username and above avatar. if you choose not to stack the will be side by side if more than one rank

Thats how I would do it anyway. Hope it helps.