For the web viewer to display images well, images need to follow a defined strategy. Strategy will depend on design of pages and may vary from the below.
Normally images placed on presentations need to be 800px to 1200px.
Normally images placed on nodes can be smaller and are typically 150px to 300px
Proportions need to be the same and follow GUI design, e.g the same rectangular form
Typically images shall bage transparent background
Minimize white areas to use as much as possible of the image area
Note that the exact sizes need to follow the customer specific design of the web viewer.
Application does not put any restrictions on image formats, but browser compatability and quality has to be taken into account. It is also possible to limit image formats in imports to block unwanted formats.
It is advisable to use common image formats which are compatible with as many browsers as possible. A special note on svg format is that it is commonly supported to a limit. Using earlier versions of svg improves compatability.