XMP, IPC and EXIF
In addition to the
longdesc attributes, the image can include the text embedded in it using XMP, IPC or EXIF fields. These can be added quickly and easily using many photo editors, including free apps and some websites. Completing the License field with a license allowing reuse will encourage others to copy the image and attribute it to your website or webpage.
example with text embedded in metadata fields:
Metadata tags with or without structured data
<meta content="Turing's universal machine was built at Bletchley Park, England, and was actually named..." />
To add this as structured data use http://schema.org/ImageObject which allows you to specify a license and author as well.
description can include the text, and
keywords can add further context.
SEO depends on if a web or image search is used
Example: 19 word quote, searched for without enclosing in quotes and as a web search
sometimes it's the people that no one imagines anything of who do the things that no one can imagine
The top results are images from pinterest and tumblr, followed by whisper. One of the top pins for the words above is which using pinterest's Visit Site gives the original source as a blogspot blog written in Jan 2015- which linked it from tumblr. The original tumblr link no longer displays on the blog (or tumblr). But the image has still got the top SEO for that phrase in only 8 months.
From looking at my own site this means an Images search with relevant text and/or phrases from the text in the description field will make the image rather than the page rank highly on google. For web rather than image-only searches pinterest.com combined with the
alt property/tag produces very high SEO. Pinterest uses
alt by default to describe your images, so each pin associates your image with that text (users can change this when they pin it though). Including the text in the
alt on a very high ranking site like pinterest is important for SEO. Either pin all images yourself, or create very pinnable images, users in Pinterest can click the image directly to visit your website.
Add the text to the image's filename SEO
In the example quote above after excluding social media images the top two results both came from images on from a page that including keywords from the quote in the page's name - http://moviezmagazine.com/2014/10/sometimes-people-one-imagines-anything-things-one-can-imagine-imitation-game/ - a similar way of naming images would increase SEO. Google's naming advice confirms this, the filename is readable even if the text isn't (yet).
Copyright violations, and pinterest tips for SEO
Pinterest works well for referrals in, but will involve multiple copies of your images appearing all over the web- regardless of any license you set. Since we are talking about text within images this should be less an issue then doing this with original art which is for sale.
You can provide a 'pin it' button which searches for your existing pins from the page rather than creating new ones. I don't recommend uploading images directly to pinterest because this won't link them to your website - instead pin direct from the site and verify your pinterest account. This should gain followers for you, and will notify them every time you pin a new image - so new images should increase hits to the page very quickly, and rise in SEO rankings quickly.
It seems likely that in time google will be able to read text inside images, just not quite yet.