Render image programmatically based on field content
You have two primary things avaiable in a node template.

content: That contains rendered markup. Use it to print stuff.

node: This contains the node object with the raw values. Use this for comparisons and accessing the unformatted values. The same raw values would also be accessible through #items in content, but it's much more complicated to access.

So in your case, access the value with node.field_resource_type.value.

Note that this contains the key of the list field, and not the displayed label. I would recommend to always specify explicit keys (e.g., put datasheet|Datasheet in the allowed values field). Then you can also reliably translate those labels if you have a multilingual site and your template doesn't break.

Unless you actually have multiple values in there, you also don't need a loop. But if you do, you can loop over it with for item in node.field_resource_type and then use item.value.

See also http://wizzlern.nl/drupal/drupal-8-entity-cheat-sheet