Learn Icelandic: Myndin virðist vera kyrr. - The picture seems to be still.

The verb virðast (to seem) triggers this structure. Instead of saying 'It seems that the picture is still', Icelandic pulls the subject of the action (Myndin) to the front, making it the main subject of virðist. The verb 'to be' becomes the infinitive vera. Crucially, because vera is a linking verb, the adjective kyrr must agree in case (nominative), gender (feminine), and number (singular) with that raised subject Myndin.

Can I rephrase this sentence using a subordinate clause instead of the infinitive?

Yes! Instead of the infinitive structure, you can use the dummy subject það and say Það virðist að myndin sé kyrr (It seems that the picture is still). Notice that when you use this structure, the verb in the að (that) clause takes the subjunctive mood (sé instead of er) because it expresses an appearance or subjective assumption rather than a confirmed fact.

Why doesn't the feminine adjective kyrr have a specific ending to match the feminine noun myndin?

The adjective kyrr (still or quiet) belongs to a small group of adjectives ending in -rr. For these adjectives, the feminine nominative singular form is identical to the masculine nominative singular form. If the subject were neuter, like barnið (the child), it would add a 't' and become Barnið virðist vera kyrrt.