In Moodle 1.8 I added a database activity to a course and I added a picture field to the only table.
Going to add a record I filled the form asking for a picture path and an alternative text.
I entered a correct path for my picture and I submitted the form.
Moodle allowed this to me.

Since Moodle 1.8 is strict if HAS TO require the alternative text. What I believe is that the alternative text should change to a red bordered field and an alert should appear.