If you really must upload PNG, then please provide SVG or high-resolution (>1000 pixels width) black-and-white PNG images. That would allow to use these images in printed form in an acceptable quality.
The English Wikipedia has detailed guidelines about their preferred format of chemical structure images: WikiProject Chemistry – structure drawing
This category has the following 17 subcategories, out of 17 total.
- ► Structural formulas created with SVG (14 C)
- ► Chemicals by shape (1 C)
- ► Hormones by chemical structure (3 C)
- ► Markush structures (42 F)
- ► Chemical structures built with molecular model sets (13 C, 1044 F)
- ► Chemical structures to explain nomenclature (340 F)
- ► Organic compounds (27 C, 57 F)
- ► Verified chemical structure diagrams (894 F)