People with low vision enlarge the font size on their screen to a readable level. However a screen magnifier is needed to enlarge bit maps. Using this can lose clarity and resolution, so that content can still be difficult to access. This becomes a true accessibility problem when the text in an image is important content. But, when the name of the site or a heading is presented only as a graphic, the user may become disorientated without this information.
Buttons are often the biggest problem, They contain important navigational instructions.
If the user has difficulty reading the text on the button, they are often left confused and stranded. So how can I put content into graphics without being less accessible. As a last resort, you can provide redundant textual content. All graphics can be problematic because they can not be rendered in an alternative presentation that is better suited for a user. Text can be read, enlarged, reduced, whatever.