Bonuses tend to take up 3 forms:
1. Squad bonus - a share of a squad-wide bonus based on team achievement (league finish, cup wins, CL qualification), split on number of performances. In football terms, it's a small number, but most of us would be more than happy.
2. Performance bonuses. These can take several forms - bonuses for appearances, clean sheets, goals / assists which are awarded match-to-match and paid monthly, or annual bonuses which may be based on hitting a cumulative target (such as 20 goals / assists - as decided by Opta). Usually players will get one or the other - match-to-match bonuses are more common for young / fringe players who may not get a lot of games whereas cumulative bonuses, which will be much bigger amounts, will apply to regular first-choice players to incentivise season-long performance. I'd expect Mo would be on a cumulative bonus, rather than match to match. Bonuses for CL qualification / competition wins can also be in individual contracts and are usually subject to minimum appearance levels (so the player has actually contributed to the success). We did have a scenario once where a player got £500k CL qualification bonus despite being out on loan all season and we learned a hard lesson that day on how to draft the clauses.
3. Loyalty bonuses - basically, when the club pays the player's agent fee (pretty standard to do that), he'll be taxed on it as a benefit and the loyalty bonus is designed to help him pay the tax when it arises. These bonuses are usually subject to him still being employed when they fall due, so they are often timed for after closure of the transfer window.
A separate matter is signing-on fees which are now rare as they need to be paid every year of the contract, including after the player has left the club. They're unconditional and expensive so rarely used.
For tax reasons, image rights payments need to be linked to commercial activities, not to football matters, so we always resisted any bonuses in those agreements, particularly if linked to playing achievements, as that would potentially make them salary payments, negating the tax advantages of the agreements for both the player and the club. It would be OK to have incremental payments for image rights if the duties went beyond the norm - that would be a personal appearance with no linkage to the club (i.e. not in red kit or similar) or if the player did something over and above what was in his normal agreement (e.g. 2-day trip to do an appearance overseas or something like that). Those payments would usually be subject to separate agreements.
