I don't think you can store data such as this in MySQL.
Well you may be able to store Binary data in SQL but filemaker won't see them as container fields.
Unless Im wrong this is a big reason to not move to Mysql.
We could talk all day about this but after many years of head wrecking I settle to the point:
Only move to SQL back end if you have a good reason to...
Other than this you are only making a lot of extra work for yourself and there doesn't seem any point in meeting trouble half ways.
I think its best to interact for Mysql for certain tasks.
E.g I have a client with a mysql based website and I use my FM system to push some product updates to the website, clean and simple.
Honestly nobody will thank you for the extra bother, not clients anyway as they don't care as long as it works.
Hope it helps.
If you are thinking of storing NO data in FM and all the data in MySQL, then that may be a bad idea. That's not what the ESS feature is about.
If you don't have the book already, get the FTS Advanced book from the FileMaker Inc website and check chapter 9 (integration). It has a great list of gotcha's, of things that behave differently when you use ESS data vs native FM data.
Only make your decision after you reviewed that.
I would not do this (use FM as front end). I integrate FM and other SQL sources, as needed, but only if needed.
However, you may consider a URI reference to images (stored where both MySQL and FileMaker can see the image) and the URI (http://...) would be TEXT in a field/column. FM can display in webviewer. MySQL would need a web interface to link to the image.
Base64 is image encoded as TEXT which can be stored in either database, but would need to be decoded back to be "image".
Neither of this may be optimal. See Wim's answer.