What is JPEG?

A lossy image format designed for photographs, where losing a little detail in a gradient is invisible.

Who uses it

Places that will not accept a PNG upload.

Writing JPEG

The same render as PNG, saved as JPEG. It is the worse choice for a table and it is worth knowing why: JPEG compresses by discarding detail around sharp edges, and a table is nothing but sharp edges. Letters get a visible halo and hairline rules blur. It also has no transparency, so a background colour is always painted rather than left clear.

What it can hold

JPEG is an image format, and the important consequence is that it is a one-way trip: JPEG compresses lossily and softens text edges, which is why a table saved as JPEG reads worse than the same table as PNG. Keep whatever you converted from, because reading the table back out of JPEG is a much harder job than writing it.

It keeps

A line break inside a cell, which survives rather than ending the row.
Presentation - weight, alignment, colour and widths - alongside the values themselves.

It cannot express

Types. Every value is text. A column of digits is indistinguishable from a column of numbers until something downstream decides to read it as one, which is where a leading zero usually disappears.
A null. An absent value and an empty one look the same, and nothing in the file says which was meant.
Nesting. Cells are flat, so structured values have to be flattened into extra columns or written into one cell as text.
A partial read. The document has to be complete to mean anything, so a truncated file yields nothing and a large one has to be held in memory whole.

Questions

Should I use JPEG or PNG for a table?

PNG, unless something you are uploading to refuses it. JPEG was designed for photographs and its compression damages exactly the kind of high-contrast edges that text and table rules are made of.

How do I open JPEG files?

JPEG is written by Tablizer rather than read by it. Open the file in whichever tool the format belongs to.

Is JPEG a good format for sharing data?

It depends who is receiving it. Places that will not accept a PNG upload. If the recipient is outside that group, a plain CSV travels further than anything else.