In common/ideas/fastnavyideas.txt :
You got the picture name wrong. Switch "GFX_idea_generic_sea_focused_navy" to "generic_sea_focused_navy", otherwise it will show up as a "?"

In common/decisions/categories
I recomend not editing 00_decision_categories.txt, you will need to update this in the future, as new categories may be added into it in future vanilla updates. Instead, create a file like "fastnavy_categories.txt" and add your categories there.