well it is a learning experience and you are far from the only one to submit something like that

if you submit a secret again, aim for legibility in the design, proofread your text and try to cut out excess wording if you can

tips:

make sure there isn't text in the background; the tv tropes page in the background wasn't necessary for your secret to get across and all the text from it competed for attention with your own text

colors: contrast is your friend; if you have dark backgrounds, use the lightest colors for your text, and viceversa

avoid neon or bright colors

there are places to look for legible and clear color combinations if you are interested

align the text in a way that it is easy to read; sure it is tempting to arrange it around your background picture, but if you find yourself having to place your text all over the place, then find a better picture! the picture should serve the secret, not the other way around

use a font size and type that is easy to read, give that priority over the prettiness of the font

save in a format that doesn't blur the text, jpg and jpeg seems to do this, in particular, but not exclusively, with red texts

avoid red text

try to aim for symmetry, in your block(s) of text, both horizontally and vertically, and leave enough space between elements (text, parts of the background picture, borders, etc.), that it looks balanced and pleasant to the eye