Alt texts can be added to every uploaded image in the Image properties ->Image info. If you have included an image in your survey via “Upload”, the “Image properties” dialog box with the “Image Info” tab will open automatically. Enter your alt text there in the corresponding “Alternative text” field.
“Alternative Text” input field for an uploaded image in the Image Properties dialog box, Image Info tab
Why are alternative texts for images valuable and necessary?
Alternative texts for images (alt texts for short) are essential for blind people and people with visual impairment because they make the content and function of the images accessible to this target audience. Alt texts can be read aloud by assistive technologies, such as a screen reader, which people with visual impairments often use. Alternative texts are thus an essential measure for making surveys with images accessible.
By the way: How and where you can insert images in the survey, you can read in our help center, How to insert images in a survey?