Problems with resizing png covers

[tzugen]

Problem description

When I dowload missing covers through the web interface, they are not shown in some places.
I found that this happens especially with png files.
When I then use my client (Ultrasonic) to request the cover art, the API returns only the cover if the size attribute is not specified.
When the size attribute is specified no cover is returned.

Presumably there is a problem with resizing non-jpeg covers.

Steps to reproduce

• Create a cover.png inside a music folder.
• Navigate to airsonic web interface. Observe that it won't get shown in the overview, but will be shown sometimes if you select the album.

System information

• Airsonic version: 10.6.2
• Operating system: Docker (linuxserver/airsonic)