The illustration of Elsa featured on her Gloves Off card is the work of Brittney Lee, a visual development artist for Disney who worked on both of the Frozen films
These will transform after you fulfill some kind of condition on the card, usually either an activated ability (i.e., paying to exile Terra, Magical Adept) or triggered ability (i.e., having six permanents in the yard for Exdeath, Void Warlock at end of turn)
Generally, the whole exercise would be quite simple, but there are three pitfalls: Some sequences of 3 are repeated on the sheet (there are three such instances on the Gym Heroes commons sheet)
Don't be fooled by it's smaller footprint
I get that TappedOut has a very active custom card community - that community is probably the most active on the site - but you have to think about the limitations of the page you are using