The rebellion led by - and named for - Engelbrecht Engelbrechtsson began in 1434. The rebels were a heterogenous lot; farmers upset over brutal sheriffs and high taxation, iron producers and merchants irritated over rising prices and the higher nobility angered by the king's appointment policies. In 1439 Erik of Pomerania was sacked as king in all three countries. The Swedes were ruled by steward Karl Knutsson Bonde until they in 1441 followed the Danes and Norwegians in chosing Christopher of Bavaria, sister-son of Erik of Pomerania, as king.
Engelbrecht Engelbrechtsson was killed in 1436, not - probably not - because of the rebellion, but as a result of an ordinary medieval feud. He has been widely used as a symbol of almost anything, by leftist revolutionary romantics to right wing nationalists.
The statue in Stockholm was raised in 1916. It is not meant to depict Engelbrecht in person, but an anonymous man in the rebellion. Proletarian writer Ivar Lo-Johansson has written that it is a remarkable statue since it shows the ordinary man; the people defending the country.
What we see is a man stringing a crossbow. The crossbow was a symbol for the youth organisation of the conservative party - there too meant to symbolise the Engelbrecht rebellion, or really freedom, via the rebellion The poster was produced as propaganda for the conservative party in 1914. The text reads "First a strong defence!" and the threatening eagle is, I suppose, meant illustrate Germany.