Hi christel150,
There isn't an instance we're aware of that would cause copying plain text to convert to an image automatically. Could you provide more information on how this is occurring, exactly?
In the meantime, here are some steps for copying and pasting on your iPhone: Select, cut, copy, and paste text on iPhone - Apple Support
"Select and edit text
- To select text, do any of the following:
- Select a word: Double-tap with one finger.
- Select a paragraph: Triple-tap with one finger.
- Select a block of text: Double-tap and hold the first word in the block, then drag to the last word.
- After selecting the text you want to revise, you can type, or tap the selection to see editing options:
- Cut: Tap Cut or pinch closed with three fingers two times.
- Copy: Tap Copy or pinch closed with three fingers.
- Paste: Tap Paste or pinch open with three fingers.
- Select All: Select all the text in the document.
- Replace: View suggested replacement text, or have Siri suggest alternative text.
- Format: Format the selected text.
- : View more options.
"
Cheers.