Images in Impress lose their text alternatives when a presentation is exported as PDF. Images inserted in presentations can have a Title and Description ("text alternatives"). This is an accessibility requirement for users of screen readers (blind users) and for exporting to non-visual formats such as audio books (e.g. in DAISY format) and Braille.

To reproduce the issue, follow these steps:
1. Create a presentation in Impress; add a picture to a slide.
2. Right-click on the picture and select Description from the context menu.
3. Add a short description in the Title field and a longer description in the Description field.
4. Save the presentation; then export it as PDF. In the PDF Options dialog, check at least "Tagged PDF" (the PDF is mostly inaccessible without those tags).
5. Open the resulting PDF file in a program that allows you to check the accessibility of the PDF, e.g. Acobe Acrobat Pro (Adobe Reader can only check whether PDF is tagged or not, it can't perform a full accessibility evaluation), the free PDF Accessibility Checker (PAC) by access-for-all.ch, or the online PDF checker at http://accessibility.egovmon.no/en/pdfcheck/. The accessibility report will say that an image is missing a text alternative.

Notes:
1. Writer does not lose text alternatives on images when exporting to PDF.
2. You can go through the above steps using three variations: give the image only a title, give the image only a description, give the image both a title and a description. In each of these cases, the images in the PDF file will have no text alternative.