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Mastodon doesn't seem to send notifications for replies without the @-tag. You might want to tag the other poster you replied to as well 
If you haven't gotten enough responses yet, you might want to reach out to the Open Source Design community. They allow posting jobs for volunteer work as well. Job posts automatically create forum posts, which I recommend participating in.
The OSS Design job board could also be engaged for the style guide and the special projects.
A designer could set a style guide with fonts and colours, and then the "help wanted" image could be replaced by the given font and a background in the given colours. The logo might also be incorporated into the image. This is less work and easy to manage. Using consistent colours and fonts with the logo also helps identify images and posts quickly. More creative work should be reserved for special posts, at which point the community, or some contractors, could be engaged again.
One location for discussion might be useful, given how vague this seems.
Are you seeking to contract someone long term? What graphics do you need? Something to grab attention or communicating specific ideas? Could it be an image created once for posts by the project to represent the project as a whole? Or a set of assets to be arranged randomly or programmatically? Could the community provide graphics as they get time, or does the post need to be hidden before adding graphics?
