Ladder and Cross
The ladder is seen leaning against the cross or integral to the cross. The ladder may be shown empty or occupied. If occupied, the figure might represent Jesus, a disciple, or a soldier.
Christ on the ladder
A figure representing Jesus, climbing willingly or unwillingly, might be shown in an abstract and metaphorical sense. Following tradition, Jesus would not have climbed a ladder to the cross; rather His hands would have been first nailed to the crossbeam (patibulum), which was then hoisted up with ropes.
Another, equally metaphorical interpretation is Christ using a ladder to ascend to heaven. In Gen. 28:10-17 we read how Jacob saw a ladder or stairway to heaven, so for a long time many religions have included an idea of such a ladder. (Even Jesus indirectly referred to something similar to Jacob's Ladder in John 1:51.)
Jesus' ascension to heaven is recorded in Mark 16:19 and elsewhere, using phrases such as "He was taken up", with nothing to suggest Jesus climbed a ladder. The important point is that he ascended to heaven. How, is not important. But an image of a ladder is a useful reminder that the means to reach heaven is still there. Jesus' ascension was not a one-off - we can all follow.1
Disciple on the ladder
Followers of Jesus are reported to have removed the body of Jesus from the cross, and for this, a ladder would almost certainly have been used although one is not specifically mentioned in the Scriptures. The followers may have been Joseph of Arimethea or perhaps Nicodemus, a prominent Jew, who climbed up to remove the nails and take away the body of Jesus (see John 19:38-42).
The ladder, nails and pincers are some several implements that make up the Arms of Christ. They all contributed to the agonizing Crucifixion, known as The Passion.
Soldier on the ladder
A sponge was soaked in vinegar and offered to Jesus (Matt. 27:48). The sponge was attached to a reed or pole, which suggests the person did not use a ladder. (The upper beam of the cross at that time is thought to have been somewhere between 2.5 to 3.5 metres from the ground.)
In John 19:32-34 we read that a soldier confirmed the death of Jesus by thrusting a spear into His side. If he had used a knife or a sword, he might have needed a ladder, but the soldier used a long-handled spear so we can speculate a ladder was not required.
The sponge and spear also feature in the Arms of Christ.
If an actual ladder is not specified, why include it in artwork?
Generally a ladder is necessary to reach places otherwise inaccessible. A ladder is not merely an aid to reach those places; it is an essential piece of equipment. And it's not a motorised lift or escalator; the ladder must be climbed.
Heaven is inaccessible without a spiritual ladder, which we must climb. Some regard religion (Mosaic Law, the Church, etc.) as this ladder, where dynamic and active membership is a requirement. Others regard religion more as an assistant, teacher and guide. Jesus taught (John 14:6), however, that the ladder is 'salvation'; and that is the meaning of the cross.
The 'however' in the previous paragraph points to a fundamental difference in ideas. The 'religion' ladder has several rungs. Artwork of a ladder leaning against a cross often has seven rungs, each representing various stages to complete before reaching heaven. Conversely, the 'salvation' ladder has just one rung, like a letter 'H'. (...and now you know what the 'H' means in 'Jesus H Christ'.)2 See also IHS Cross.