Fix thumbnail issues

[m-mohr]

• Add missing await
• Correctly parse bbox if it has 6 elements
• pass img instead of url

ToDo: Check why images get shown too large

[DanielJDufour]

Changes look good. I'm not sure it'll completely solve the smearing, but it's a big step in the right direction :-)

[StijnCaerts]

Can this be merged? I think this may cause thumbnails not to be rendered on an Item in stac-browser if they are not the first 'candidate'.

[m-mohr]

No, there's still the open question: "Check why images get shown too large"

Also, merging this won't be available in STAC Browser unless someone works on a PR to upgrade from stac-layer 0.15 to 1.0 (or provides funding to STAC Browser and this library).