Episodes 2, 4, 6, 10: Improving image descriptions (#922)

* Episode 2: improving image descriptions (issue #781)
  - Description suggestion for the 'python-zero-index.svg' figure.

* Episode 4: improving image descriptions (issue #781)
  - Description suggestion for the '03-loop_2_0.png' figure.

* Episode 6: improving image descriptions (issue #781)
  - Add description suggestion for the second and third iterations of the 'for loop' output figures.

* Episode 10: improving image descriptions (issue #781)
  - Add description suggestion for 'python-overlapping-ranges' figure.

* Fix long lines

* Episode 2: Changes requested to #922.
  - Modify alt-text in 'python-zero-index.svg' to match suggestions from #844.

Author: vinisalazar

_episodes/02-numpy.md

@@ -233,7 +233,11 @@ It takes a bit of getting used to,
 but one way to remember the rule is that
 the index is how many steps we have to take from the start to get the item we want.
 
-![Zero Index](../fig/python-zero-index.svg)
+!["data" is a 3 by 3 numpy array containing row 0: ['A', 'B', 'C'], row 1: ['D', 'E', 'F'], and
+row 2: ['G', 'H', 'I']. Starting in the upper left hand corner, data[0, 0] = 'A', data[0, 1] = 'B',
+data[0, 2] = 'C', data[1, 0] = 'D', data[1, 1] = 'E', data[1, 2] = 'F', data[2, 0] = 'G',
+data[2, 1] = 'H', and data[2, 2] = 'I',
+in the bottom right hand corner.](../fig/python-zero-index.svg)
 
 > ## In the Corner
 >

_episodes/04-loop.md

@@ -18,7 +18,7 @@ keypoints:
 In the last episode, we wrote Python code that plots values of interest from our first
 inflammation dataset (`inflammation-01.csv`), which revealed some suspicious features in it.
 
-![Analysis of inflammation-01.csv](../fig/03-loop_2_0.png)
+![Line graphs showing average, maximum and minimum inflammation across all patients over a 40-day period.](../fig/03-loop_2_0.png)
 
 We have a dozen data sets right now, though, and more on the way.
 We want to create plots for all of our data sets with a single statement.

_episodes/06-files.md

@@ -86,22 +86,26 @@ inflammation-01.csv
 {: .output}
 
 ![Output from the first iteration of the for loop. Three line graphs showing the daily average,
-maximum and minimum inflammation over a 40-day period for the patients in inflammation-01.csv.](
+maximum and minimum inflammation over a 40-day period for all patients in the first dataset.](
 ../fig/03-loop_49_1.png)
 
 ~~~
 inflammation-02.csv
 ~~~
 {: .output}
 
-![Analysis of inflammation-02.csv](../fig/03-loop_49_3.png)
+![Output from the second iteration of the for loop. Three line graphs showing the daily average,
+maximum and minimum inflammation over a 40-day period for all patients in the second
+dataset.](../fig/03-loop_49_3.png)
 
 ~~~
 inflammation-03.csv
 ~~~
 {: .output}
 
-![Analysis of inflammation-03.csv](../fig/03-loop_49_5.png)
+![Output from the third iteration of the for loop. Three line graphs showing the daily average,
+maximum and minimum inflammation over a 40-day period for all patients in the third
+dataset.](../fig/03-loop_49_5.png)
 
 Sure enough,
 the maxima of the first two data sets show exactly the same ramp as the first,

_episodes/10-defensive.md

@@ -277,7 +277,8 @@ The range of each time series is represented as a pair of numbers,
 which are the time the interval started and ended.
 The output is the largest range that they all include:
 
-![Overlapping Ranges](../fig/python-overlapping-ranges.svg)
+![Graph showing three number lines and, at the bottom,
+the interval that they overlap.](../fig/python-overlapping-ranges.svg)
 
 Most novice programmers would solve this problem like this: