Making text easy to read
Text needs contrast with what is behind it. White text with a black outline reads well on almost any photo, which is why it is the classic meme style. On a plain light area, dark text without an outline looks cleaner. Keep captions short: fewer words at a larger size beat a paragraph in small type.
Long text wraps onto several lines automatically and stays inside the edges of the image. Turn ALL CAPS off for a calmer look.
Fonts on different devices
The Bold meme style uses Impact where your device has it, as Windows and Mac do. Phones without it use the boldest similar font available. Clean and Classic use your device's own sans-serif and serif fonts.