Add missing descriptions for Image's documentation #19333

Job	Run time
📊 Static checks / Code style, file formatting, and docs	29s
🍎 macOS / Editor (target=editor, tests=yes)	5m 16s
🍎 macOS / Template (target=template_release)	3m 15s
🌐 Web / Template (target=template_release)	1m 10s
🤖 Android / Template (target=template_release)	3m 19s
🍏 iOS / Template (target=template_release)	2m 37s
🐧 Linux / Editor w/ Mono (target=editor)	3m 8s
🏁 Windows / Editor (target=editor, tests=yes)	8m 12s
🐧 Linux / Editor with doubles and GCC sanitizers (target=editor, tests=yes, dev_build=yes, scu_build=yes, precision=double, use_asan=yes, use_ubsan=yes, linker=gold)	12m 11s
🏁 Windows / Template (target=template_release)	7m 19s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	5m 58s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	6m 41s
🐧 Linux / Template w/ Mono (target=template_release)	1m 2s
🐧 Linux / Minimal template (target=template_release, everything disabled)	41s
🪲 Godot CPP / Build and test Godot CPP	6m 52s
	1h 8m 10s

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