Error: Area too small but image is there

[FloC3]

I have a slider with images. They are normal HTML and not background-image. But every image throws an error: Skipping image - XXXX - Area too small

the area settings are these:
bottom: 409.46875
height: 0
imageHeight: -1
imageLeft: 0
imageTop: 199
imageWidth: 600
left: 37.375
right: 338.125
top: 309.46875
width: 300.75

it seems like the width could be calculated but not the height. Why? The image URL in the log message is correct. When I click it, I see my picture...

additional:
the images in the slider are rendered smaller than their original size. But the BackgroundCheck takes the original size. This would lead to a wrong detection of the slider arrows I think. Do I have to tell the BackgroundCheck some settings, to take the correct image size? I could not check it because of the above error.