If I understood correctly, you have an image, you do "copy" then "paste as brush" and you get that error.
I tried to do the same:
1-open an image (in my case a flower in .png)
1a-scaled down to around 200 px because it was huge
2-copy
3-paste as brush
4-entered brush name and filename ("white-rose"), letting space to default 25
5-entered OK
I have the new brush in my set
I'm using last samj's 2.10.4
I tried to do the same:
1-open an image (in my case a flower in .png)
1a-scaled down to around 200 px because it was huge
2-copy
3-paste as brush
4-entered brush name and filename ("white-rose"), letting space to default 25
5-entered OK
I have the new brush in my set
I'm using last samj's 2.10.4