Learn Icelandic: Ég ætla að mála verkstæðið sjálfur í dag. - I am going to paint the workshop myself today.

Even though it sits next to the object verkstæðið (the workshop), sjálfur (myself) is emphasizing the subject of the sentence, ég (I). Because ég is doing the action and is in the nominative case, sjálfur must also take the nominative case to match it.

The word sjálfur is masculine. Does this mean the speaker is male?

Yes. The pronoun sjálfur changes to match the gender of the person or thing it refers to. Since it is sjálfur here, the speaker is a man. A female speaker would say sjálf, and someone using gender-neutral language would use the neuter sjálft.

Could I put sjálfur right next to ég instead of later in the sentence?

In Icelandic, it is much more natural to place sjálfur after the object (verkstæðið) when you just mean you will do the task yourself instead of having someone else do it. Putting it right after the pronoun, like Ég sjálfur ætla..., sounds extremely dramatic and poetic, similar to saying "I, myself, shall..." in English.

Just to review, why does the word verkstæðið end with a -ð here?

The base word is verkstæði (workshop), which is a neuter noun. The -ð at the end is the attached definite article, turning it into "the workshop". It is in the accusative case here because it is the object being painted, but remember that for all neuter words, the accusative form looks exactly the same as the nominative form.