The print on Franky's shirt reads something like "I☀HT". I Googled about it and nothing came up. Also, I have checked the One Piece Wikia page and it doesn't have any details about the logo. What does it mean and does it have any significance?
Some comment here suggests that the sun is a reference to the rising sun, so "AM", as in "in the morning"/"Ante Meridian". It then becomes "I AM HT". HT would be the acronym of 変態 (HenTai) or pervert.
So that would mean the T-shirt is actually subtly saying "I am a pervert".